Why is the speed of light constant towards all observers?

[PokePat321]

I'm making a project for school about time dilation, however I would need proof light has the same velocity to all observers. I know it's a postulate, however I couldn't find any experiments that prove this. I would like to have this knowledge for myself alongside the people I'll be presenting it to.

[ajb]

The constant speed of light (in vacuum) for all inertial observers can be seen in the wave solutions to Maxwell's equations for the electromagnetic field. This I believe was the real starting place for special relativity as this constant speed cannot be understood in the framework of Newtonian-Galilean mechanics.

 

As for experimental proof lots of physics indirectly tests the speed of light. For example all the collider experiments at CERN show that our mathematical descriptions of particle physics that require the constant speed of light are consistent with nature.

 

The classical experiment that attempted to measure the change of the speed of light due to the Earth's movement through a supposed aether is the Michelson-Morley experiment. They can up with a null result here and this was also very important in the development of special relativity. Modern experiments now are focused on testing what we call Lorentz invariance, which is really the mathematical structure of special relativity, and all results so far support the constant speed of light. Google the MM experiment.

 

If you look at the wikipedia page http://en.wikipedia.org/wiki/Speed_of_light they list some methods of measuring the speed of light. Have a look at these.

Edited January 29, 2014 by ajb

[swansont]

The constant speed of light (in vacuum) for all inertial observers can be seen in the wave solutions to Maxwell's equations for the electromagnetic field. This I believe was the real starting place for special relativity as this constant speed cannot be understood in the framework of Newtonian-Galilean mechanics.

 

 

And this is something many people fail to realize — Maxwell's equations predicted the speed of EM waves are independent of the source or receiver speed (one has to realize that visible light is an EM wave for this to "click" — that wasn't known right away). So if all you want to do is show that c is the same for a moving observer, show examples of moving sources or receivers still able to emit/receive EM signals. We see light from distant objects that are red- or blue-shifted, but it's still light. Space probes can still send and receive signals from us, despite our relative motion. A radio works when you carry it across the room.

[Enthalpy]

If light had had a speed =c in one special frame of reference, and had differed in others, we would just have written Maxwell's equation differently.

 

These equations would have additional terms to compensate for the speed versus the absolute frame of reference.

 

For me, the invariant speed of light is strictly experimental.

[Sensei]

For me, the invariant speed of light is strictly experimental.

 

Indeed. But human made devices never fly with v>30,000 m/s (or so). That's 0.1% of c. Voyager-1 has v=17,000 m/s relative to Sun.

[imatfaal]

If light had had a speed =c in one special frame of reference, and had differed in others, we would just have written Maxwell's equation differently.

 

These equations would have additional terms to compensate for the speed versus the absolute frame of reference.

 

For me, the invariant speed of light is strictly experimental.

 

 

No - really not the case. Firstly there is no "Maxwell's equation" there are four equations with the names of the great and good of electricity and magnetism attached to them. If you set charge density rho to zero and current J to zero, taking the curl of the curl equations (Ampere/Maxwell and Faraday/Maxwell as the others are set to zero Gauss Law and Magnetism) and a bit of heavy duty rearrangement later you have the wave equations second order differential equations linking E field and B field. The constant is one over the permitivity times the permeability of the vacuum - this comes from Amperes law of the circuit - the only solution will be with a constant speed of light equal to one over the sqrt of those two natural constants.

[swansont]

If light had had a speed =c in one special frame of reference, and had differed in others, we would just have written Maxwell's equation differently.

 

These equations would have additional terms to compensate for the speed versus the absolute frame of reference.

 

For me, the invariant speed of light is strictly experimental.

 

If physics were different, we would have different equations. Yes. It's a tautology, which doesn't really shed much light on anything, nor does it address the OP. Also, you don't get your own version of physics. Whether you choose to acknowledge it or not, invariant c is embedded in Maxwell's equations. Which means that experiments that rely on the resulting wave equation are a confirmation that the notion is correct.

[Enthalpy]

Yes, I prefer the wording "experiments confirm the notion is correct" to something like "the constant speed results from the equations".

[StringJunky]

Yes, I prefer the wording "experiments confirm the notion is correct" to something like "the constant speed results from the equations".

"The constant speed results from the equations" answers the OP better than "experiments confirm the notion is correct" since the question is about why and not whether it has been confirmed.

[swansont]

I pointed out that it's from Maxwell's equations because of the remark that it's a postulate of SR. That's true, but it's important to note that it was not simply conjured up. E&M has an invariant speed of propagation of EM radiation. SR was an attempt to look at the ramifications for kinematics.

 

The equations do not exist in a vacuum. They are built on experimental result, as well as theoretical development, which (as this is a fairly mature theory/model) go hand-in-hand. So IMO, "experiments confirm the equations" and "it's a prediction of the equations" are two sides of the same coin. Use whichever one makes you most comfortable.

 

As for the OP, it asks what experimental evidence confirms an invariant c. Most experiments that involves EM radiation and relative motion should suffice. If c is not invariant, you don't end up with the wave equation. If one is so inclined, one is free to investigate what you'd get if c was not invariant and find out how badly you would break E&M.

[studiot]

PokePat has noted this is a school project so it is good to pitch the level of answer there.

 

It has been mentioned that the speed of light is the constant for all observers and sources.

 

This is true but it is worth noting that it is a characteristic of (classical) wave motion that the speed of the wave is independent of the source.

That is the speed depends only upon the medium.

 

What is different for light is that the speed is also constant for all material observers. The word material is also important.

 

As to why, that is a philosophical rather than a physics question

[Delbert]

The constant speed of light (in vacuum) for all inertial observers can be seen in the wave solutions to Maxwell's equations for the electromagnetic field. This I believe was the real starting place for special relativity as this constant speed cannot be understood in the framework of Newtonian-Galilean mechanics.

Maxwell's equations are based of physics. In that simple statement it is clear the speed of light will be the same for everybody, whatever speed you think you, or anybody else you're observing, are doing.

 

It seems to me that with light (or perhaps anything else) there has to be a thing that we call or experience as speed of travel. For example, if light travelled instantaneously, then upon switching on (say) a torch the light would instantaneously reach to an infinite distance. And presumably light covering an infinite distance would require or need an infinite amount of energy so to do, which is ludicrous. So therefore, light must be observed to travel at some sort of identifiable speed based on physics, to avoid a ridiculous situation. In other words it appears its speed of travel is determined by a fundamental property of physics, not to mention the universe. Relative speed doesn't come into it, therefore the speed of light will always be the same whatever you're or observing.

 

I recall on another science site someone arguing that gravity acts instantaneously. You know, if (say) the Sun's gravity varied we would feel it here on Earth at the same time and not approximately 8 minutes later like light. Well, it seems to me the same restriction and ludicrousness about an instant effect over distance applies to gravity as it does light - in other words gravity travels at a speed according to physics, producing gravity waves as a consequence.

Edited February 13, 2014 by Delbert

[swansont]

 

I recall on another science site someone arguing that gravity acts instantaneously. You know, if (say) the Sun's gravity varied we would feel it here on Earth at the same time and not approximately 8 minutes later like light. Well, it seems to me the same restriction and ludicrousness about an instant effect over distance applies to gravity as it does light - in other words gravity travels at a speed according to physics, producing gravity waves as a consequence.

 

That's a shortcoming of Newtonian physics. In GR, changes in gravity propagate at c.

[ajb]

Maxwell's equations are based of physics. In that simple statement it is clear the speed of light will be the same for everybody, whatever speed you think you, or anybody else you're observing, are doing.

There are always some subtleties here, all inertial observers measure the (local) speed of light to be c.

 

And presumably light covering an infinite distance would require or need an infinite amount of energy so to do, which is ludicrous. So therefore, light must be observed to travel at some sort of identifiable speed based on physics, to avoid a ridiculous situation.

That is an interesting argument, but only suggests that light cannot travel at an infinite speed as measured by any and thus all inertial observers. It does not, as far as I can see suggest a constant speed of light.

[Schneibster]

The cause is the exact hyperbolic relation between the time dimension and each of the three space dimensions.

 

The proof of the constancy of the speed of light is that the theory of relativity correctly predicts the lifetimes of variously moving cosmic rays and their decay products. There are other proofs but that one is the most important in this realm.

 

I recommend this Baez page: http://math.ucr.edu/home/baez/symmetries.html

Edited February 20, 2014 by Schneibster